This project investigates the neural mechanisms that subserve the monkey's ability to discriminate noxious thermal stimulation. In the present investigation, nociceptive neurons were identified in the primary somatosensory cortex (SI) of the anesthetized monkey. The receptive fields of nociceptive neurons were classified as either wide dynamic range (WDR), preferentially responsive to noxious mechanical stimulation, or as nociceptive specific (NS)< responsive to only noxious stimuli. Nociceptive SI neurons were somatotopically organized. Receptive fields on the foot were located more medial than those with receptive fields on the hand. WDR neurons were distributed over a wider area of SI than were NS neurons. NS neurons were only found in the anterior half of the distribution of WDR neurons. An examination of the neuronal stimulus-response functions obtained during noxious thermal stimulation of the foot or the hand indicated that WDR neurons exhibited significantly greater sensitivity to noxious thermal stimulation than did NS neurons. The majority of neurons were located in the middle layers (III & IV) and tended to be found in clusters. The differences in responses properties and the locations of WDR and NS neurons suggest that they may subserve different functions. WDR and NS neurons may provide information concerning transient changes in noxious thermal stimulation. In addition, NS neurons may be involved in the process of localization and cortical activation that accompany novious thermal stimulation.